Shark population in decline near populated islands in Pacific

The survey by the University of Hawaii showed that the numbers were drastically lower near populated islands in Hawaii, the Mariana Archipelago and American Samoa, compared to more pristine, remote areas in the ocean.

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“We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90% compared to those at the most untouched reefs,” said Marc Nadon, lead author of the study in the journal Conservation Biology.

“In short, people and sharks don’t mix,” added Nadon, a scientist at the university’s Joint Institute for Marine and Atmospheric Research.

The latest research was based on a method called “towed-diver surveys,” in which paired scuba divers record shark sightings while being towed behind a small boat.

“Towed-diver surveys are key to our effort to quantify reef shark abundance,” said Ivor Williams, head of the team responsible for the surveys.

“Unlike other underwater census methods, which are typically at an insufficient spatial scale to properly count large, mobile species, these surveys allowed our scientists to quickly record shark numbers over large areas of reef.”

Researchers analyzed data from over 1,600 towed-diver surveys taken from 2004-2010, in combination with information on human population growth and reef area, as well as satellite-data on sea surface temperature and ocean traits.

“Around each of the heavily populated areas we surveyed — in the main Hawaiian Islands, the Mariana Archipelago and American Samoa — reef shark numbers were greatly depressed compared to reefs in the same regions that were simply further away from humans,” Nadon said in a statement.

“We estimate that less than 10% of the baseline numbers remain in these areas.”

Researchers counted five types of reef sharks for the study, including the most common types — gray and whitetip reef sharks — as well as blacktip reef sharks, Galapagos sharks and nurse sharks.

“The pattern — of very low reef shark numbers near inhabited islands — was remarkably consistent, irrespective of ocean conditions or region,” said co-author Julia Baum, assistant professor at the University of Victoria, Canada.

“Wherever there are people there are not reef sharks, so by implication they must have declined. But we can’t say what period that decline happened over,” she said in an interview with AFP.

“What they really like is really productive waters, so waters that are rich in phytoplankton because that high productivity means… a richer prey base for them,” Baum added.

“There are also a lot more reef sharks in areas where it is warm.”

Co-authors came from the University of California, Santa Barbara; the Centre for Conservation Research in Calgary, Canada; Simon Fraser University; Scripps Institution of Oceanography and the US National Oceanic and Atmospheric Administration.